Staff: Mentor

Stimulated emission only occurs in lasers, as it takes very specific circumstances to occur. (Or rather it only occurs in large quantities in a laser. It has a small chance of happening in nature)

The amplification by stimulated emission in a laser refers to light of a certain frequency causing excited electrons to fall to a lower energy state and emit another photon of the same frequency as the first. So the original beam of photons is quickly reinforced and amplified by additional photons.

I believe the mirrors bounce the light back and forth allowing it to stimulate many electrons before passing through one and emerging from the laser as a collimated beam. My own laser pointer has a lens that focuses the beam at a specific distance. (I know because I just lost the lens this weekend. Now I don't have a "dot" but instead I have a much larger "spot" that grows larger the further away a surface is.)

When a photon traveling along the axis of the laser tube strikes an excited atom, a second photon of the same vector and frequency is emitted. The mirrors are the feedback/gain mechanism that allows this avalanche process to continue. The front mirror also called the output coupler is not a complete mirror, and so lets some of the photons out.
The collimation is based on the dimensions of the tube assembly, and the lasing medium.